This invention relates to electronic weighing apparatus of the type utilizing a strain gage load cell as the active element which provides the electrical signal indicative of the weight of an article.
Recent advances in the force measurement field have taken place primarily due to the improvement in the operating characteristics of the strain gage load cell. The load cell is a transducer which when subjected to certain forces provides an accurate electrical indication of the force or forces to be measured. These versatile transducers are being utilized in an ever increasing number of applications due to several important operating characteristics including the insensitivity of the load cell to off-center loading.
The ability of the load cell to measure weight directly and accurately has resulted in their use in electronic scales wherein compression loads from the weighing platform are applied to the load receiving surface of the load cell. One advantage to the electronic weighing apparatus is the absence of flexures, pivots and adjustments and, as a result, the weighing platform is directly coupled to the load receiving surface of the load cell.
The weighing platform of the apparatus is normally required to have a large surface area so that bulky articles can be readily accommodated thereon. However, the load receiving surface of a load cell may be less than one square inch so that the weighing platform and the supporting frame therefore extend outwardly a substantial distance from the load cell. The supporting frame for the weighing platform provides the needed support through a number of force translating arms which extend outwardly from the load cell to the peripheral region of the weighing platform.
When the article to be weighed is placed in the central portion of the weighing platform, the load is shared by the supporting arms. To provide protection to the load cell in the situation where the article exceeds the measuring capacity of the load cell, stops have been located near the periphery of the weighing platform so that upon a predetermined deflection by the applied weight the stop limits further deflection.
However, the placement of the article upon the peripheral portion of the weighing platform generally results in an unequal sharing of the force by the supporting arms and in certain situations, one may translate the entire force to the load cell. Consequently, the rated load in this situation results in substantially greater platform deflection than the placement of the rated load in the center of the platform. To provide overload protection for the condition wherein the entire rated load is dropped by the operator at the center of the platform, the gaps between the stops and the underside of the weighing platform are substantially reduced thereby derating the scale.
Further protection for the load cell has been provided by the incorporation of a protective structure within the load cell. This typically is the use of a gapped structure for the cell body with the gap size being determined by the largest static load to be expected. Since the weighing apparatus has been found to experience dynamic loads during operation which are significantly higher than the static protection provided, the failure rate of load cells in weighing apparatus increases. The dynamic condition, wherein an article with the rated capacity of the apparatus is dropped on the weighing platforms, provides a high impulse causing the stress to build quickly and then almost instantaneously reverse subjecting the structure to a load greatly exceeding that of the static load counterpart. Thus, external stops and the derating of the weighing apparatus are normally utilized in combination with the internal protection.
Additional measures taken to decrease the failure rate of load cell weighing apparatus have utilized a stiffer and stronger load cell therein. While this approach has tended to reduce the failure rate, the system is less sensitive to a given load and consequently a lower electrical signal is generated by the commercially available strain gages affixed to a stiffer load cell structure. Since the load cell output signal is in the millivolt range, the lower signal level is undersirable due to the lower signal to noise ratio and the fact that the sensitivity of succeeding electronic devices is normally required to be increased.
It is desired to increase the sensitivity of load cells without requiring the use of internal stop mechanisms contained within the structure of the load cell. Internal stop means normally introduce unwanted and unpredictable variations in the output signals. To enhance sensitivity, load cells are designed to permit increased deflection and to rely on the weighing apparatus to include the external stopping mechanisms for limiting the deflection under different loading conditions.
The optimization of load cell deflection requires consideration of conflicting requirements. As the deflection of the load cell increases, the setting of external overload stop gaps becomes less critical but structural-linearities increase due to the increased torque within the structure. As a result, the sensitivity of the load cell to off-center loading increases. This is similar to the non-linearity in performance exhibited by load cells containing internal gaps for overload protection. Further, the natural frequency of the apparatus decreases as the permitted deflection increases. In industrial environments the unbalanced rotors of electric motors running at 1800 rpm established a predominant resonance frequency of 30 Hz. This factor establishes an upper limit for permissable deflection. Thus, the overload protection provided by a weighing apparatus should enable the mechanical vibrations in the environment to be accounted for while at the same time permitting practical lower limits on deflection.
Accordingly, the present invention is directed to a strain gage load cell weighing apparatus wherein improved overload protection for static and dynamic loading is provided. The overload protection enables the derating of load cell and scale capacity to be substantially reduced and, further, the use of the sensitive measuring instrument itself for overload protection is not required.